Cable Sheath Fault Testing | 7500V Murray Bridge Cable Fault Locator Pinpoints

The XHHG521A Cable Outer Sheath Fault Tester is designed on the Murray bridge principle and is used to locate the breakdown points of various wires and cables, as well as defect points that have not yet broken down but show an abnormally low insulation resistance.

The equipment adopts a motor-driven voltage regulator and an R-type transformer to form a high-voltage constant-current source, and the voltage is raised and lowered by key operation. It provides a no-load voltage of 7500V and a short-circuit current of 100mA, and uses a high-sensitivity amplifier and a galvanometer to indicate balance, forming a balanced bridge together with the ratio potentiometer; the whole unit is placed at a high potential. The measuring cable is a specially designed twin-core high-voltage rubber cable, and a four-terminal resistance measurement method avoids the error introduced by the lead resistance. The panel as a whole stays at a low potential. With one-key operation, the fault distance is calculated automatically once the cable length is entered. The high-voltage source and the bridge are integrated in a portable protective case, which makes the equipment high in voltage, light in weight, easy to operate and safe to use.

This tester is built specifically for cable outer sheath fault testing and for breakdown-point location on installed cables, rather than for general-purpose conductor testing. Its bridge-balance method has no blind zone and it computes the fault distance automatically once the cable length is entered, so it suits users who need fast, accurate and repeatable on-site results. It is especially suited to:

For users who need to locate the breakdown points of cables after they have been laid, in particular high-resistance breakdown points that are difficult to burn into low resistance, such as a linear high-resistance breakdown at a cable intermediate joint.

For users who deal with flashover-type breakdown points and with defect points that have not yet broken down but have a low resistance, such as insulation defects that a megohmmeter shows to be of low resistance but that do not break down under operating voltage.

One-key operation and automatic fault-distance calculation keep the workflow simple, while the automatic fault-point testing function is retained together with the manual testing function, so both routine and on-site measurement needs are covered.

The cable fault locating bridge method works as follows: short-circuit the fault phase and the non-fault phase of the cable under test; connect the two arms of the bridge to the fault phase and the non-fault phase respectively; adjust one variable resistor on the two arms so that the bridge is balanced; then, by the proportional relationship and the known cable length, the fault distance can be obtained. The advantages of the bridge method are that it is relatively simple and its accuracy meets the requirements of on-site engineering tests, and that a two-phase short-circuit fault of a cable line is very convenient to measure. Using the Murray bridge to locate the breakdown point is a classic method that is convenient and accurate. The basis of the bridge method is that the resistance of the core (or shielding layer) is uniform and proportional to the length.

If the distances from the two ends of the tested cable to the breakdown point are L1 and L2, the total cable length is L, and the corresponding core resistances are R1 and R2, then after the cable is connected to the bridge a balanced circuit is formed; with r1 + r2 = r0, the balance condition gives L1 = P%·L, where P is the balance adjustment reading.